A grinding process is performed on the circumferential surfaces of metal annular members such as the raceway surfaces of race members of a radial rolling bearing, or cylindrical shaped fitting surfaces in order to improve the surface precision or surface roughness. As illustrated in FIG. 1A and FIG. 1B, this grinding process is performed using a grinding device. The grinding device normally comprises: a rotating drive shaft (not illustrated in the figures); a backing plate 2 that is fastened to the tip-end section of the rotating drive shaft, and that magnetically affixes an annular processed object (work) 1 on the end surface thereof; at least two shoes 3 for making it possible to position the processed object in the radial direction; a rotating grindstone 4 that grinds the outer-circumferential surface or inner-circumferential surface of the processed object 1; a contact sensor or non-contact sensor (not illustrated in the figures) that comprises at least two gauge heads 5 and that measures the outer diameter D of the processed object 1; and a controller (not illustrated in the figures) for controlling the feed speed of the grindstone 4 based on the obtained measurement results. The grinding process is performed in a state in which the processed object 1 is positioned in the radial direction by supporting and fastening the processed object 1 to the backing plate 2, and bringing the shoes 3 into sliding contact with the outer-circumferential surface of the processed object 1. At the same time, the feed speed of the grindstone 4 is suitably controlled by measuring the outer diameter D at two locations on opposite sides in the radial direction of the outer-circumferential surface of the processed object 1 that is being ground in process by the sensor and feeding the measurement results back to the controller.
More specifically, the grinding of a metal annular member is performed in the order of rough grinding, finish grinding and spark out. In rough grinding and finish grinding, the feed speed of the grindstone 4 (feed amount/time) is reduced in stages. Then, when the outer diameter D of the processed object 1 that is calculated based on the values measured by the gauge heads 5 becomes a target dimension, the feed speed of the grindstone 4 is set to “0”, and spark out begins. Spark out is a process of grinding the circumferential surface of the processed object 1 in a state in which the feed speed of the grindstone 4 is “0”, and by only the circumferential surface of the processed object 1 pressing against the surface of the grindstone 4 due to elastic restoration of the processed object 1. After a specified amount of time (sufficient time for the sparks and grinding sound from the contact area between the outer-circumferential surface of the processed object 1 and the surface of the grindstone 4 to stop, and for the outer-circumferential surface of the processed object 1 to become smooth) has elapsed, the grindstone 4 is caused to displace in a direction away from the outer-circumferential surface of the processed object 1, and grinding ends.
In this kind of grinding process, the processed object 1 is elastically deformed from the state illustrated in FIG. 7A to an elliptical shape as exaggeratedly illustrated in FIG. 7B by pressing the grindstone 4 against the processed object 1 in the rough grinding process, and the processed object 1 is elastically restored in the processing after that (finish grinding and spark out). The elastic deformation of the processed object 1 becomes more prominent the lower the rigidity is of the processed object 1. Here, in order to avoid interference with the grindstone 4, the installation positions of the gauge heads 5 are normally shifted about 90° in the circumferential direction from the contact position between the processed object 1 and the grindstone 4. Therefore, the outer diameter D of the processed object 1 that is measured by the gauge heads 5 becomes larger than the outer diameter in the free state (state in which the outer-circumferential surface is not pressed by the shoes 3 and grindstone 4, and elastic deformation is released). As a result, by performing spark out after the outer diameter D of the processed object 1 has been processed to the target dimension, the amount of grinding becomes excessive, and the outer diameter in the free state of the processed object 1 that is obtained becomes smaller than the target dimension.
For example, it can be considered to make the feed speed of the grindstone 4 “0” in a state in which the outer diameter D of the processed object 1 that is measured by the gauge heads 5 is larger than the original target dimension and perform spark out for a specified amount of time such that the outer diameter in the free state of the processed object 1 may be made as a target dimension. However, the amount of elastic deformation of the processed object 1 fluctuates due to the cutting ability of the grindstone 4 and the like. In other words, as the cutting ability of the grindstone 4 degrades, the amount of elastic deformation of the processed object 1 becomes larger, and the better the cutting ability of the grindstone 4 is, the smaller the amount of elastic deformation of the processed object 1 becomes.
Therefore, even when spark out is performed for just a specified amount of time from a state in which the outer diameter D of the processed object 1 is larger than the target dimension, when the cutting ability of the grindstone 4 is worse than a set value, and the amount of elastic deformation of the processed object 1 is greater than a set value, the amount of elastic restoration of the processed object 1 becomes large, and the amount of grinding during spark out becomes excessive, so the outer diameter D of the processed object 1 becomes smaller than the target dimension. Conversely, when the cutting ability of the grindstone 4 is better than a set value, the amount of elastic restoration of the processed object 1 during spark out becomes less than a set value, and the amount of grinding of the outer-circumferential surface of the processed object 1 by spark out becomes less than a set value, so the outer diameter D in the state after grinding of the processed object 1 is complete becomes larger than the target dimension. When the outer diameter D at the end of spark out of the processed object 1 is larger than the target dimension, it is feasible to make the outer diameter of the processed object 1 the target dimension by further performing step-feed grinding of the processed object 1 as illustrated in FIG. 8 in which the feed speed of the grindstone 4 and the amount of cutting are very small. However, by adding this process, there is a possibility that the time and work of processing, and the manufacturing cost will increase.
Moreover, when the amount of elastic deformation of the processed object 1 is less than a set value, and the amount of elastic restoration of the processed object during spark out is small, the time required for releasing the elastic deformation of the processed object 1 during spark out will be short. However, in the case of conventional construction, the amount of time that spark out is performed is set, based on the set value of the cutting ability of the grindstone 4, to an amount of time sufficient for sparks and grinding noise from the area of contact between the outer-circumferential surface of the processed object 1 and the surface of the grindstone 4 to stop, and for the outer-circumferential surface of the processed object 1 to become smooth. Therefore, when the cutting ability of the grindstone 4 is better than a set value, the amount of time that spark out is performed and the overall amount of time that grinding is performed becomes unnecessarily long.
In regard to this, JP 2000-343425 (A) discloses a method of learning the starting point for spark out (timing at which grinding changes from finish grinding to spark out) from the amount of change in the outer diameter per one rotation of the processed object 1 at the end of spark out, and then adjusting the starting point of spark out in the next grinding after this learning is complete. Moreover, JP 2012-143843 (A) discloses a method of adjusting the feed speed of the grindstone in the next grinding process that is performed after that learning based on the amount of time required for making the outer diameter of the processed object the target dimension. However, in the case of the methods disclosed in the literature above, when variation occurs in the grinding process due to changes in the cutting ability of the grindstone and the like, there is a possibility that the learning will not converge.